1. Related Field
The present invention relates generally to data transmission wherein digital signals are multiplexed and demultiplexed. More particularly the invention relates to a data transmission system according to the preamble of claim 1 and a method according to the preamble of claim 14. The invention also relates to a computer program according to claim 24 and a computer readable medium according to claim 25.
2. Description of Related Art
In telecommunication systems of Synchronous Digital Hierarchy (SDH) type, the synchronization signals may be carried by so-called E1 signals (2.048 Mbit/s). Similarly, in Synchronous Optical Networking (SONET) systems, the synchronization signals may be carried by so-called T1 signals (1.544 Mbit/s). In both cases it is the inherent clock frequency of the signals E1/T1 (i.e. 2.048 MHz and 1.544 MHz respectively) that carries the synchronization information through the systems.
A telecommunication system normally uses atomic clocks to create the synchronization signals. These signals (e.g. of E1 type) are then transported through the network, and far out in the network the signals may be fed to a base station having a radio interface. In such a case, the synchronization signals will control the radio frequencies transmitted by the base station for communication with for example cell phones. Hence, even very small fluctuations of the clock frequencies, may cause substantial performance problems. To avoid this kind of problems, the wander of the synchronization signals must be lower than predefined limits, e.g. as specified in the standards ITU-T G.823 and ITU-T G.813.
Typically, each mobile operator distributes a separate clock signal in its network. A so-called backhaul operator may provide network resources for two or more mobile operators. This means that in a given physical network signals originating from different clock sources may have to co-exist. Furthermore, multiple mobile operators may sometimes share a specific base station site. Such a site is a location, e.g. a tower, where a plurality of base stations may be installed. A site sharing situation may arise when one backhaul operator serves a number of mobile operators via one base station (or cell) site. Here, the technical problem for the backhaul operator is to send the different synchronization signals, together with the traffic signals, to the respective mobile operator at the cell site as efficiently as possible.
Let us assume that E1 signals are used. Then, each mobile operator uses his own atomic clock to generate all the E1 signals in his system. Since these E1 signals all originate from the same source (in most cases an atomic clock), a group of E1 signals from a given mobile operator may be referred to as a particular synchronization group.
For example, an STM-1 signal of 155.52 Mbps may carry up to 63 E1 signals and all these E1 signals would then belong to the same synchronization group. The atomic clock for this synchronization group controls the frequency corresponding to the bit rate, namely 155.52 MHz. The 155.52 MHz frequency will therefore be extremely exact because it originates from an exact clock. The inherent frequency of the E1 signals will be 2.048 MHz and this frequency will also be extremely exact due to that 2.048 MHz is exactly 16×155.52/1215=2.048 MHz. This means that if for example the 155.52 MHz is multiplied with 16 (e.g. with a phase locked loop, PLL) and divided by 1215, an exact clock frequency of 2.048 MHz would be generated. Alternatively, the frequency 2.048 MHz can be generated directly from 155.52 MHz by using so called fractional division.
Nevertheless, it has proven to be very difficult to transmit two or more data or clock signals on a multiplex format with good synchronization quality/phase accuracy via a common medium. This is especially true if the signals have the same nominal frequencies, however where the signals show slight frequency deviations relative to one another. In particular, difficulties are here encountered in the demultiplexing process, where the wander must be sufficiently low, for instance to meet the requirements of given a telecom standard, such as ITU-T G.813. The term “wander” is defined as low frequency jitter, normally up to 10 Hz.
When transferring a single signal, so-called low-factor oversampling may be employed to preserve phase information. By low-factor oversampling is here understood a factor higher than one, however typically lower than two. U.S. Pat. No. 3,819,853, U.S. Pat. No. 4,920,545 and U.S. Pat. No. 6,009,109 show different solutions of this type. Unfortunately, neither of these approaches as such can be used to tackle the above-mentioned problem.
In the prior art, the problem has instead been avoided by embedding the necessary synchronization signals in the packets of Ethernet streams. Thereby, it has been possible to reconstruct an original signal on a receiver side, and thus through the use of packets, emulate a so-called circuit connection. This strategy is often referred to as “Pseudo Wire” or the high precision time synchronization protocol IEEE 1588 V.2.
U.S. Pat. No. 4,873,684 shows system for multiplexing, transmitting and demultiplexing signals having different frequencies. Here, a reference sample signal is used, which is obtained by multiplying a frequency equal to, or higher than, the frequency of the maximum frequency among the transmitted signals with the number of transmitted signals. Each signal to be transmitted is sampled based on the reference sample signal before being multiplexed into the time division format. Any empty time slots are filled with a dummy signal. Consequently, the frequency requirements may become extreme, and substantial bandwidth resources risk being wasted.
US 2008/0025346 describes a solution for synchronizing and multiplexing asynchronous signals. Here, so-called frame phase absorption is carried out with respect to the incoming asynchronous signals. As a result, synchronous signals are generated to which pointer values are assigned that describe the asynchronous properties. The synchronous signals are then multiplexed through processing of changing pointer values by a pointer transmission section.
US 2002/0018493 reveals a digital data transmission system, wherein a plurality of data signals are embedded in a carrier signal using a time division multiplex (TDM) operation. Rate matching is here undertaken between the data signals and the carrier signal by means of stuff locations. Data to be stuffed and the management information for the reassignment are embedded in a path layer overhead of the carrier signal superframe.
U.S. Pat. No. 6,888,826 discloses a solution, which enables multiple clock signals to share processing resources. Here, pointers are stored in FIFO buffers, and this in turn, renders it possible to compensate for timing differences between a system clock and the respective outgoing line clock signals, so that the clock signals can be regenerated on the receiver side.
US 2005/0078683 describes a data communication system for transferring one or more payload streamed data signals and an auxiliary data signal via a common medium. The auxiliary data signal is organized as data packets, and a transmission data formatter formats these packets into a streamed data signal format.
Then, the signal is multiplexed with the payload streamed data signals into a bit stream for transmission.
Despite the various TDM-based approaches described above, there is no prior solution enabling the transmission of two or more data or clock signals on a multiplex format via a common medium with a sufficiently high phase accuracy to meet the wander requirements of today's most important telecom standards, if said signals are based on different synchronization sources.